1. Field of the Invention
The present invention relates to ink jet printing and more particularly relates to ink droplet sensing method and apparatus.
2. Prior Art
As is known in the art, ink jet printing is a form of non-impact printing wherein ink droplets are caused to impinge upon a recording medium such as paper or the like. A typical ink jet system includes a droplet generator which starts a stream of ink along an initial trajectory, a transport for moving paper past the generator to allow droplets to impinge upon the paper, and a means for directing the ink droplets to impinge upon specific paper locations, thereby encoding the paper with information. According to one ink jet system, ink droplets are continuously generated at a fixed frequency along a trajectory toward the recording medium. The continuous drop system also includes a gutter to catch those droplets which are not to impact the paper. The trajectory of the droplets is determined at the point of droplet break-off by placing a selected net charge on the droplet. In this way, some droplets contact the paper at appropriate positions and other droplets strike the gutter and are recirculated for subsequent use.
Within the so called continuous drop subset of ink jet printers, there exists a variety of architectures. One architecture, for example, comprises a single drop generator nozzle which traverses back and forth across the ink jet page as droplets are generated. Other architectures include multiple nozzles each of which direct ink droplets to selected portions of a paper width.
One multiple nozzle architecture known in the art makes particular use of the present droplet sensing method and apparatus. According to that architecture, the droplets are charged both positively and negatively to varying degrees depending upon the desired droplet trajectory. A deflection field located downstream from the droplet break-off point interacts with the positively and negatively charged droplets to deflect them away from their initial trajectory in a direction transverse to a direction of paper movement. Each of the multiple ink jet nozzles of this system throws droplets to a specific portion of the paper width and when the printer is functioning properly, the ink droplets from the multiple nozzles "stitch" together at stitch points across this width. In this system, the gutter droplets are more highly charged so that they deflect a large amount away from their initial trajectory to gutters comprising part of the drop deflection apparatus. Further details regarding this type of ink jet printer quite similar to this architecture can be obtained by reference to U.S. Pat. No. 4,238,804 to Warren entitled "Stitching Method and Apparatus For Multiple Nozzle Ink Jet Printer".
As droplets are generated and deflected along varying trajectories to the paper or the gutter, there is a need to check the performance of the ink jet generator, charging electrode and deflection field. This checking process must be performed periodically so that the calibration of the printing system never deteriorates. One particularly important feature in the ink jet process is the stitching together of drops from the nozzles along the ink jet array. Neither droplet overlap nor gaps between nozzles can be permitted if the ink jet printing image is to be uninterrupted across the paper width. For this reason, it is important that some sensing of droplet trajectory be conducted to insure that droplets are responding to the charging and deflection in a desired manner.
Prior art drop sensors are known. One such sensor is disclosed in U.S. Pat. No. 4,255,754 to Crean et al assigned to the Xerox Corporation, assignee of the present invention and incorporated herein by reference. Apparatus disclosed in that patent includes a light source and light sensor mounted on opposite sides of a droplet trajectory. The sensor interprets changes in light intensity transmitted by the light source to sense both the presence of ink droplets and to determine their speed of travel to the recording medium. U.S. Pat. No. 4,344,078 to Houston filed Nov. 6, 1980 and issued on Aug. 10, 1982 discloses one method for practicing the sensing technique disclosed in the Crean et al patent. According to the method disclosed in that application, the light source and light receiver include optical paths which are photo-fabricated to a support substrate. The present invention comprises an alternate drop sensing method and apparatus which offers ease in sensor fabrication and improvements in drop sensing sensitivity.